How to Use the Mortgage Paydown Calculator

A field-by-field guide — what each input means, where to find the value, and how it affects your results.

The Mortgage Paydown Calculator has four sections: Loan Details, Recurring Extra Payments, Ad-Hoc Extra Repayments, and Ad-Hoc Rate Changes. Only the Loan Details section is required — everything else is optional and lets you model more specific scenarios.

Section 1 — Loan Details

These four fields describe your mortgage as it currently stands. You can find all of them on your most recent loan statement or in your lender's online banking portal.

Initial Loan Amount

This is the original amount you borrowed — not your current remaining balance. If your loan was for $600,000, enter 600000 even if you have already paid some of it down.

Where to find it:Your loan contract, or your lender's app under “loan details” or “original loan amount”.

Why it matters: The calculator uses this together with your start date and interest rate to reconstruct your full amortization schedule from the beginning. This is what allows it to accurately compare your current trajectory against a plan with extra repayments.

Example: You bought a home and borrowed $580,000. You've been paying for 2 years and now owe $550,000. Enter 580000, not 550000.

Loan Term (Years)

The total length of your loan in years as agreed when you took it out. Most Australian home loans are 25 or 30 years.

Where to find it:Your loan contract or your lender's app. It's usually labelled “loan term” or “remaining term”.

Why it matters: The loan term determines your baseline minimum monthly repayment. A longer term means smaller minimum repayments but far more interest paid over the life of the loan.

Tip: If your lender shows a remaining term (e.g., “27 years left”), use the original term instead (e.g., 30 years), and match it with the original start date. The calculator handles time elapsed automatically.

Initial Interest Rate (%)

Your current annual interest rate, entered as a percentage. Enter 6.29 for 6.29% p.a. — do not convert it to a decimal.

Where to find it:Your most recent loan statement, lender app, or mortgage documents. Look for “interest rate”, “variable rate”, or “fixed rate”.

Why it matters: The interest rate is the single biggest driver of how much you pay over the life of your loan. Even a 0.5% difference can change your total interest by tens of thousands of dollars. If your rate has changed since you took the loan out, enter your current rate here and add the original (or intermediate) rate as a rate change back-dated to your start date.

Variable vs fixed: If you are on a fixed rate that expires soon, enter your current fixed rate now. Once you know your new rate, come back and add a rate change for the date your fixed period ends.

Loan Start Date

The date your first repayment was due — or the date your loan was drawn down (settlement date). This is usually the first of the month after settlement.

Where to find it: Your loan contract, settlement statement, or first loan statement.

Why it matters: Combined with the loan term, this tells the calculator exactly where you are in your repayment schedule today — and therefore how much principal and interest remain.

Example: You settled on 15 March 2022 and your first repayment was due 1 April 2022. Enter 2022-04-01.

Section 2 — Recurring Extra Payments (Optional)

This section lets you model a regular extra amount you plan to pay on top of your minimum repayment — for example, an extra $200 every month. Tick the checkbox to expand the fields.

This is the most common way people accelerate their mortgage. Even a modest regular extra payment has a compounding effect: every dollar of extra principal you pay reduces the balance that interest is calculated on next month, which in turn means more of your minimum repayment goes to principal the following month, and so on.

Amount

The extra dollar amount per payment period on top of your minimum repayment. This is in addition to — not instead of — your regular mortgage repayment.

There is no minimum or maximum. Start with whatever feels achievable. Even $50 per month adds up significantly over a 30-year loan.

Example: Your minimum repayment is $2,800/month. You can afford to pay $3,000/month. Enter 200 as the extra amount.

Frequency

How often you make the extra payment. Three options are available:

Monthly — the extra amount is applied every month.
Quarterly — the extra amount is applied once every three months, on the same month you set as the start date.
Annually — the extra amount is applied once per year, on the month matching the start date.

Tip:If you receive an annual bonus each July, set the frequency to “Annually” and the start date to July of this year. For your regular salary top-up, use “Monthly”.

Start Date

The date from which the recurring extra payment begins. This must be after your loan start date.

You can set this to today, a future date, or a past date (if you have already been making extra payments and want to model your full picture). For quarterly and annual frequencies, the calculator uses this month as the anchor — extra payments occur on the same month each quarter or year.

End Date (Optional)

If your extra payments will stop at a known date — for example, when a child starts school, or when a fixed expense ends — enter that date here. Leave it blank to model the extra payment continuing for the rest of the loan.

Example: You plan to pay an extra $300/month until you return from parental leave in March 2027. Set the end date to 2027-03-01.

Section 3 — Ad-Hoc Extra Repayments (Optional)

Ad-hoc repayments are one-off lump sum payments on specific dates — for example, putting your tax refund, a work bonus, or an inheritance directly onto your mortgage.

You can add as many ad-hoc repayments as you like. Each row has two fields:

Date

The date the lump sum payment will be (or was) made. The calculator applies it to the mortgage balance at that point in time, which then reduces all future interest calculations.

Lump sum payments are most effective early in the loan when the balance is highest — but they always save interest regardless of when they are made.

Amount

The dollar amount of the one-off payment. This must be a positive number. There is no upper limit — if you are modelling a large windfall, enter the full amount.

Example: You expect a $15,000 tax refund in July 2025 and plan to put it straight onto your mortgage. Add a row with date 2025-07-01 and amount 15000.

Section 4 — Ad-Hoc Rate Changes (Optional)

This section lets you model future or historical interest rate changes. Each row represents a date on which your rate changed (or will change) and the new rate that applies from that point forward.

When a rate change is applied, the calculator recalculates your minimum monthly repayment for the remaining loan term at the new rate — exactly as your lender would do. This lets you model scenarios like a rate cut later this year, or the end of a fixed-rate period.

Date

The date the new interest rate takes effect. This must be after the loan start date.

For a past rate change you want to model, use the actual date it occurred. For a future rate change — for example, if your fixed rate expires next year — use your best estimate of the effective date.

New Rate (%)

The new annual interest rate as a percentage from that date onward. Enter 5.75 for 5.75% p.a.

Example: Your 2-year fixed rate of 5.99% ends on 1 February 2026, and you expect to revert to a variable rate of 6.49%. Add a row with date 2026-02-01 and new rate 6.49.

Multiple rate changes: You can add as many rate change rows as you like. The calculator applies them in chronological order. This is useful for modelling several RBA rate decisions over the coming years.

What Happens When You Calculate

Once you press Calculate My Paydown Plan, the calculator generates a complete month-by-month amortization schedule for two scenarios side by side:

Your baseline — paying minimum repayments only, with any rate changes applied.
Your plan — the same loan with your extra repayments and rate changes applied.

The results show you the total interest saved, the number of months (and years) saved, and a chart of both balance curves over time. You can then share your plan as a URL — all your inputs are encoded in the link so anyone you share it with sees exactly the same calculation.

For more on what the results mean, see the Mortgage FAQ.

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